Morphology of river bed scours on a delta plain revealed by interferometric sonar

High-resolution bathymetric data from an interferometric sonar were used to investigate scours within the distributary channels of the Øyeren delta plain, the largest freshwater delta in northern Europe.The data reveal a range of scours occurring at different settings, including channel confluences, bends, confinements and bedrock scours. The basal erosion surfaces of the scours are up to 23 m below the local base level (i.e. lake level), and up to 4 times greater than the mean channel depth. This significant relief on the basal erosion surface is assigned to autocyclic processes like scouring intrinsic to the river channels, and the effect of changes in local base level is considered to be insignificant in this respect. The deepest scour is 24 m deep and is located at a channel confinement. It probably formed due to flow convergence during one flood cycle, although once formed it has been relatively stable over several years. River bend scours have been registered to a depth of 23 m on the delta plain. Channel confluences are shown to change in planform morphology due to different confluence angles. They are up to 13 m deep, and channel depth is deeper downstream of the scour than upstream.